[configtls] add new config to specify TLS curve preferences (#12174)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description
Allow users to mention their preferred curve types for ECDHE handshake.
Add a new config option for this.
This will provide users with more control over specific settings during
the handshake process.

<!--Describe what testing was performed and which tests were added.-->
#### Testing
Added

<!--Describe the documentation added.-->
#### Documentation
Updated readme<!--Please delete paragraphs that you did not use before
submitting.-->

Note: Please let me know if I need to open an issue for this.

Author: VihasMakwana

.chloggen/add-curve-preferences-tls.yaml

@@ -0,0 +1,25 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+component: configtls
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Allow users to mention their preferred curve types for ECDHE handshake
+
+# One or more tracking issues or pull requests related to the change
+issues: [12174]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

config/configtls/README.md

@@ -14,6 +14,12 @@ By default, TLS is enabled:
   the exporter's HTTPs or gRPC connection. See
   [grpc.WithInsecure()](https://godoc.org/google.golang.org/grpc#WithInsecure)
   for gRPC.
+- `curve_preferences` (default = []): specify your curve preferences  that will
+	 be used in an ECDHE handshake, in preference order. Accepted values are:
+  - X25519
+  - P521
+  - P256
+  - P384
 
 As a result, the following parameters are also required:
 

config/configtls/configtls.go

@@ -70,6 +70,11 @@ type Config struct {
 	// ReloadInterval specifies the duration after which the certificate will be reloaded
 	// If not set, it will never be reloaded (optional)
 	ReloadInterval time.Duration `mapstructure:"reload_interval"`
+
+	// contains the elliptic curves that will be used in
+	// an ECDHE handshake, in preference order
+	// Defaults to empty list and "crypto/tls" defaults are used, internally.
+	CurvePreferences []string `mapstructure:"curve_preferences"`
 }
 
 // NewDefaultConfig creates a new TLSSetting with any default values set.
@@ -231,6 +236,14 @@ func (c Config) loadTLSConfig() (*tls.Config, error) {
 	if err != nil {
 		return nil, err
 	}
+	curvePreferences := make([]tls.CurveID, 0, len(c.CurvePreferences))
+	for _, curve := range c.CurvePreferences {
+		curveID, ok := tlsCurveTypes[curve]
+		if !ok {
+			return nil, fmt.Errorf("invalid curve type: %s. Expected values are [P-256, P-384, P-521, X25519]", curveID)
+		}
+		curvePreferences = append(curvePreferences, curveID)
+	}
 
 	return &tls.Config{
 		RootCAs:              certPool,
@@ -239,6 +252,7 @@ func (c Config) loadTLSConfig() (*tls.Config, error) {
 		MinVersion:           minTLS,
 		MaxVersion:           maxTLS,
 		CipherSuites:         cipherSuites,
+		CurvePreferences:     curvePreferences,
 	}, nil
 }
 
@@ -448,3 +462,10 @@ var tlsVersions = map[string]uint16{
 	"1.2": tls.VersionTLS12,
 	"1.3": tls.VersionTLS13,
 }
+
+var tlsCurveTypes = map[string]tls.CurveID{
+	"P256":   tls.CurveP256,
+	"P384":   tls.CurveP384,
+	"P521":   tls.CurveP521,
+	"X25519": tls.X25519,
+}

config/configtls/configtls_test.go

@@ -871,3 +871,53 @@ func TestSystemCertPool_loadCert(t *testing.T) {
 		})
 	}
 }
+
+func TestCurvePreferences(t *testing.T) {
+	tests := []struct {
+		name             string
+		preferences      []string
+		expectedCurveIDs []tls.CurveID
+		expectedErr      string
+	}{
+		{
+			name:             "X25519",
+			preferences:      []string{"X25519"},
+			expectedCurveIDs: []tls.CurveID{tls.X25519},
+		},
+		{
+			name:             "P521",
+			preferences:      []string{"P521"},
+			expectedCurveIDs: []tls.CurveID{tls.CurveP521},
+		},
+		{
+			name:             "P-256",
+			preferences:      []string{"P256"},
+			expectedCurveIDs: []tls.CurveID{tls.CurveP256},
+		},
+		{
+			name:             "multiple",
+			preferences:      []string{"P256", "P521", "X25519"},
+			expectedCurveIDs: []tls.CurveID{tls.CurveP256, tls.CurveP521, tls.X25519},
+		},
+		{
+			name:             "invalid-curve",
+			preferences:      []string{"P25223236"},
+			expectedCurveIDs: []tls.CurveID{},
+			expectedErr:      "invalid curve type",
+		},
+	}
+	for _, test := range tests {
+		tlsSetting := ClientConfig{
+			Config: Config{
+				CurvePreferences: test.preferences,
+			},
+		}
+		config, err := tlsSetting.LoadTLSConfig(context.Background())
+		if test.expectedErr == "" {
+			require.NoError(t, err)
+			require.ElementsMatchf(t, test.expectedCurveIDs, config.CurvePreferences, "expected %v, got %v", test.expectedCurveIDs, config.CurvePreferences)
+		} else {
+			require.ErrorContains(t, err, test.expectedErr)
+		}
+	}
+}